Asphaltene is defined as the fraction of crude oil insoluble in n-heptane, and composed of nitrogen, oxygen, and sulphur atoms in addition to carbon and hydrogen. Asphaltenes are polar molecules which aggregate together through aromatic .pi.--.pi. orbital association, hydrogen bonding, and acid-base interactions. At the molecular level, asphaltenes consist of n-alkyl chains attached to aromatic rings and n-alkyl bridges connecting two aromatic rings. In both cases, the alkyl group distribution is predominated by the lower hydrocarbon chain lengths.
Asphaltenes generally deposit on pore-containing clay materials in the form of solid deposits or dark sludges. The solid deposits probably result from the growth of asphaltene aggregates on existing surfaces while the sludges are probably formed as large aggregates in solution, which settle out and drag other components out with them. These latter deposits cause serious problems anywhere in the oil production circuit.
In the formation, asphaltene deposition affects oil production in several ways. Pore throats become blocked thus reducing permeability. The formation can also become more oil-wet, changing the relative-permeability relationships. Asphaltene may promote water-in-oil emulsions, leading to much higher viscosities. In the wellbore, asphaltene deposits gradually reduce the area for flow. This results in the need for higher pressure drops to maintain production at an acceptable level. Deposition in downhole safety valves may interfere with their proper operation. Build up in the tubing may damage wireline tools or require different running procedures.
Heavy oils typically contain higher quantities of asphaltenes than conventional oils. Some heavy oils are mobile at reservoir conditions and are produced by primary recovery processes. Most asphaltene deposits in heavy oil are found in unconsolidated formations. Wells that are producing heavy oils often encounter operational problems including well sanding, and rod and tubing parting. Hence, these wells require frequent workovers. Usually, a condensate blend, i.e., a mixture of light hydrocarbons, is used as a workover fluid. Some wells lose their productivity immediately after such workover operation, because the light hydrocarbons in the condensate blend dissolve the heavy oil which may contain up to 20% asphaltenes. This dissolution of heavy oil into the condensate blend disturbs the equilibrium conditions of the asphaltene molecules, which ultimately precipitate. The presence of mobile formation sand around wellbores, along with sludge-type asphaltene precipitates, creates conditions leading to severe near-wellbore formation damage, and eventual termination of the well.
Many oil companies are looking at chemical alternatives to clean up deposits such as asphaltene deposits and prevent their formation. The solvents used most frequently are toluene and xylene. In recent years, research has been looking for alternative solutions because of increasing environmental restrictions and costs related to the use of these solvents. The most common procedure presently used for cleaning facilities and downhole tubing is mechanical scraping. However, it is obvious that this method is totally inappropriate for near-wellbore formation rejuvenation.
Acidizing wells for cleanup or stimulation purposes has resulted in the precipitation of asphaltenes. The pH variation changes the local chemical equilibrium, and causes an elevation of the concentration of some ions, such as iron. The predominant form of asphaltic deposit found after acidizing is iron/asphaltene sludge.
Enhanced oil recovery techniques have also led to asphaltene deposition problems. The use of water floods or CO.sub.2 floods upsets the internal balance within the native oil, while brine increases solids precipitation. The use of CO.sub.2 can support asphaltene solubility to a certain extent in dead oil. However, in a live oil, which contains solution gas, CO.sub.2 destabilizes asphaltenes.
It has also been proposed to use co-solvents for asphaltene removal and prevention of re-occurrence. The preferred solvent was described as a xylene-enriched material with water-wetting properties using moderate-length carbon-chain alcohols. Production recovery was comparable to xylene alone and production improvement was stable for several months (see Society of Petroleum Engineers, paper #21038, 1991, 393).
The use of diesel solvents such as "DP-40" and "DC-40" is described in Can. J. Chem. Eng., 1985, 63, 878. These substances do not however have the solvating power of xylene, but they resolve local problems without drawing asphaltene deposits from other areas to the area to be cleaned up.
The solvating powers of gas oil, high-aromatic diesel fuel and low-aromatic diesel fuel were also compared to that of toluene. Without co-solvents, the high-aromatic diesel fuel had a higher threshold to flocculation (i.e., more solvent could be added to the oil before the onset of flocculation) than that of toluene; the gas oil had a threshold only slightly lower than that of toluene. The use of additives did not enhance the solvating power of the diesel fuel. In studying the effects of light gases on asphaltene precipitation from heavy oils, it was noted that any precipitated asphaltene could be re-dissolved by adding more heavy oil (J. Can. Pet. Tech., 1992, 31, 24). However, this would be possible only if the heavy oil was undersaturated with asphaltene materials.
Various processes have been proposed in the patent literature to overcome asphaltene related problems. For example in Soviet Union patent 1,680,748, the use of a mixture of methanol, colophony, and surfactant is disclosed to remove asphaltene, resin, and paraffin deposits.
In Soviet Union patent 1,682,374, the use of a mixture of salt deposits inhibitor, non-ionic surfactant, hydrochloric acid, and water was suggested for asphaltene-resin-paraffin removal.
In Soviet Union patent 1,677,050, it is stated that a mixture of vat residue from butyl alcohol production (containing butyl esters, butanol butyrates, di- and polyalcohols, butyrals, monoglycol ethers, and high-boiling-point oxygen) and heavy pyrolytic resin successfully removed asphaltene-resin-paraffin deposits.
In U.S. Pat. No. 5,104,556, a mixture of kerosene and alkyl phenol is claimed to be effective in dissolving asphaltenic and paraffinic deposits. Similar disclosure of a mixture of kerosene, chromium anhydride, lower alcohol, and water is found in Soviet Union patent 1,629,493.
Soviet Union patent 1,609,807 describes the removal of asphalt, tar, and paraffin deposits from oil wells using a mixture of gas benzene, oxylated alkylphenol, glycerine, polyacrylamide, and water.
In Soviet Union patent 1,592,478, injection of a hydrocarbon solution of benzene and a specified acidic hydrocarbon emulsion is suggested for asphaltene-resin removal from the wellbore.
In U.S. Pat. No. 5,139,088, asphaltene precipitation in the flow path of an oil production well is claimed to be inhibited by injecting a heavy fraction of crude oil having a relatively high aromaticity and molar weight. The heavy fraction is obtained by separating the light fraction containing gaseous and oil components having a relatively low aromaticity and molar weight from the produced oil stream. However, the previously precipitated asphaltene can only be solubilized in the heavy fraction if such heavy fraction is undersaturated in asphaltene component.
In U.S. Pat. No. 4,465,138, a cyclic thermal solvent recovery method utilizing visbroken produced crude is disclosed. In this method, heavy oil is subjected to visbreaking operation to produce lower viscosity heavy oil. Subsequently, the hot visbroken oil is injected into the formation as a solvent and allowed to undergo a soaking period, and the well is subsequently returned to production. This method relates to the enhanced oil recovery (EOR) technique. The primary objective of this process is to reduce in situ heavy oil viscosity by injecting hot visbroken heavy oil instead of chemical solvents which are expensive and environmentally unfriendly.
In view of the foregoing, it becomes obvious that there is a great need for an improved method permitting the revival of wells closed due to wellbore or near-wellbore obstruction caused by asphaltene deposits. It would therefore be highly desirable to develop a method of dissolving asphaltene deposits which is not harmful to the environment. Such method would take advantage of the relative solubility of asphaltene in crude oil in order to restore oil production from a closed well in a short period of time.